It is necessary for wireless transmission to sufficiently compensate for nonlinear distortion generated in power amplifiers when appropriately transmitting amplitude-varying signals using a linear modulation scheme. Digital predistortion is a technique for canceling distortion produced in a power amplifier by adding an inverse distortion component to the signal input to the power amplifier. In order to achieve a satisfactory compensation effect, the amplitude and the phase of the distortion component to be added to the input signal have to be controlled at high accuracy.
One method for realizing the predistortion is using a lookup-table type predistorter configured to look for an appropriate distortion component from the lookup table corresponding to the input signal. This method is described in H. Girard and K. Feher, “A New Baseband Linearizer for More Efficient Utilization of Earth Station Amplifiers Used for QPSK Transmission”, IEEE J. Select Areas Commun., Vol.SAC-1, No. 1, 1983.
From the viewpoint of achieving more accurate distortion compensation, a power series predistorter that represents the nonlinear distortion characteristic of the power amplifier using a power series model is known. See, for example, Okamoto, Nojima, and Ohoyama, “Analysis and Compensation of nonlinear distortion in a travelling-wave tube amplifier based on IF Band Predistortion”, IEICE Technical Study Report, MW76-112, 1976.
U.S. Pat. No. 5,164,678 issued to Puri et al, entitled “Process for Compensating nonlinearities in an Amplifier Circuit” discloses automatic control for a power-series predistorter. In this publication, the output signal from the power amplifier and the respective degrees of distortion components generated by a digital predistorter are subjected to fast Fourier Transform (FFT) to perform frequency conversion, and the coefficients of the respective degrees are estimated.
Similarly, G. Lazzarin, S. Pupolin, and A. Sarti, “Nonlinearity Compensation in Digital Radio Systems”, IEEE Trans. Commun., Vol.42, No. 2/3/4, February/March/April, 1994 discloses a technique for controlling polynomial coefficients of a digital predistorter. In this publication, a covariance matrix is calculated for the signal generated by the digital predistorter, and the difference between the output signal of the power amplifier and the signal generated by the digital predistorter is used as an error to control the polynomial coefficients of the predistorter.
Another publication, T. Nojima and T. Konno, “Cuber Predistortion Linearizer for Relay Equipment in 800 MHz Band Land Mobile Telephone System”, IEEE Trans. Vech. Tech., Vol.VT-34, No. 4, Nov.1985, discloses automatic control of a power-series predistorter. In this publication, the predistorter is controlled using pilot signals in certain carrier frequencies so as to allow the polynomial to follow change in temperature or change over time in the power amplifier. This technique is practically applied to transmission amplifiers of boosters for automobile telephones.
Conventional power-series type predistorters can achieve satisfactory nonlinearity (or distortion) compensation if a sufficient amount of output backoff is guaranteed, as illustrated in FIG. 1A, or if a narrow band modulation wave is used. However, in order to operate the power amplifier more efficiently, the output backoff has to be compressed. Consequently, the predistorter is required to have an improved ability to perform distortion compensation so as to guarantee linear operation at higher input power levels.
FIG. 2 is a chart showing an experimental result measuring the relative phase of a third-order distortion component as a function of output level of a power amplifier. In the experiment, a pair of fundamental waves (or carrier waves) 102 and 104 with a center frequency f0, as illustrated in FIG. 1B, are input to the power amplifier at various power levels, and the output signals are measured. In addition to the amplified fundamental waves 102 and 104, third-order distortion components (nonlinear components) 106 and 108 appear in the output signal from the power amplifier. Usually, the third and higher distortions are generated; however, only the third-order distortion components are illustrated in FIG. 1B for the sake of simplification.
The two plots 202 and 204 shown in the chart of FIG. 2 correspond to the lower part third-order distortion 106 and the upper part third-order distortion 108 shown in FIG. 1B, respectively. Ideally, these two plots are consistent with each other over the entire output power range. If these two components agree with each other, compensating for one of the third-order distortion leads directly to compensation for the other (paired) distortion component. In contrast, if the two components do not agree with each other, a nonlinear component still remains in the signal unless both distortion components are compensated for.
In general, these two distortion components are close to each other at a low power level (for example, at or below 20 dBm), as illustrated in FIG. 2. This result agrees with the presumption that satisfactory distortion compensating effect can be achieved if a sufficient amount of output backoff is guaranteed. In contrast, if the output power level increases, the two plots 202 and 204 do not agree with each other, which means that compensation of distortion components becomes difficult in a range in which the output backoff is insufficient. The value of the third or higher order distortion component varies depending on frequency. This phenomenon is known as the “memory effect”. A method for excluding the memory effect using a time-varying filter model is described in H. Ku, D. McKinley and J. S. Kenny, “Quantifying Memory Effects in RF Power Amplifiers”, IEEE Transactions on Microwave Theory and Techniques, Vol.50, No. 12, pp.2843-2849, December 2002.
Meanwhile, the input signal being input to the predistorter has a certain degree of randomness, and accordingly, the memory effect may vary in response to the input signal varying over time. In other words, the frequency-dependent nonlinearity may vary over time. However, the conventional predistorters cannot follow such a change over time satisfactorily, and consideration of highly precise nonlinearity compensation has not been made sufficiently.
It may be proposed to cause the predistorter to follow the change over time in the distortion component using a pilot signal. In this case, the distortion component has to be compensated for using a pilot signal within, for example, the period of training sequence, independently from the signal transmission. However, because the pilot signal cannot always be acquired, it is difficult to easily and accurately compensate for the distortion using a pilot signal. In addition, compensating for the distortion using a pilot signal includes many steps, such as inputting a prescribed pilot signal to the predistorter, supplying the output of the predistorter to the power amplifier, scanning the entire frequency range to detect nonlinear distortion components, and controlling various parameters so as to reduce the detected distortion components. Accordingly, the process and the structure may become complicated.
Since it is proposed to use broadband modulation signals in the near future for wireless communication systems, highly precise compensation for distortion components is required for broad band signals over several tens of megahertz (MHz). As the frequency range to be used increases, the change in the frequency-dependent nonlinear distortion components is likely to increase, and therefore, the problem will become more serious.